More fully kill running processes on timeout

[Keno]

Killing rr doesn't actually terminate all the processes it starts. I see two options:

1. Put rr and everything under it in its own pid namespace, and then we kill the pid namespace init when we want to exit
2. Put everything into a pid cgroup, then when we want to kill things, lower the pid limit to 0 to prevent further process spawning and then start iterating through the pids in the cgroup, killing them off one by one.

[DilumAluthge]

I believe that with Buildkite, this will be resolved, because on Buildkite, at the end of the job, we destroy the sandbox, which should also destroy all of the processes.

@staticfloat Can you confirm that this is correct?

[staticfloat]

Yes, confirmed.